Method for operating an internal combustion engine especially for motor vehicles

ABSTRACT

The invention relates to a method for operating an internal combustion engine, especially for motor vehicles, the engine having a catalytic converter; in the method, for heating up the catalytic converter, there is a switchover between a homogeneous operating mode with a one-time injection of fuel into a combustion chamber of the engine and an operating mode with subdivided injection of fuel at at least two injection time points into the combustion chamber of the engine; wherein, for subdivided injections, both injection time points lie ahead of an ignition of an air/fuel mixture, the first injection time point essentially corresponds to the injection time point of the homogeneous operating mode during the switchover operation from the homogeneous operating mode to the operating mode with subdivided injection, and the second injection time point of the subdivided injection takes place at first so early that the mixture arising during operation with subdivided injection corresponds approximately to a homogeneous mixture and, after the completed switchover, the second injection time point is shifted toward late until a pregiven mixture preparation is present and, for a switchover from the operating mode with the subdivided injection to the homogeneous operating mode, the shift of the second injection time point takes place in the opposite direction. Furthermore, the invention relates to a computer program which is suitable to carry out the method when the method is carried out on a computer.

STATE OF THE ART

[0001] The invention relates to a method for operating an internalcombustion engine, especially for motor vehicles. In the method, it isprovided for heating a catalytic converter that there is a switchoverbetween a homogeneous operating mode with a one-time injection and anoperating mode with a subdivided injection of fuel at at least twoinjection time points into a combustion chamber of the internalcombustion engine. In the subdivided injection, both injection pointslie forward of an ignition of an air/fuel mixture.

[0002] A method of this kind is known, for example, from DE 101 00 682.9wherein a method is described for heating a catalytic converter ininternal combustion engines having gasoline-direct injection with thesteps:

[0003] shifting the ignition to “retard”;

[0004] checking whether the charge of the cylinders with air exceeds apregiven threshold;

[0005] subdividing the fuel injection into two component quantitieswhich are injected before the ignition when the air charge exceeds thethreshold.

[0006] Vehicles having internal combustion engines require catalyticconverters in the exhaust-gas system for exhaust-gas purification. Thesecatalytic converters must be brought to the operating temperature asfast as possible after a cold start so that means for heating areprovided. For example, after a cold start, the catalytic converter canbe heated via high exhaust-gas temperatures. This so-called motoriccatalytic converter heating has the advantage that it can be donewithout additional components.

[0007] In internal combustion engines, the exhaust-gas temperature can,in principle, be increased in that the degree of efficiency ofcombustion is deteriorated. A deterioration of the degree of efficiencyof the motoric combustion can, for example, be brought about by adeviation of the ignition time point from the optimal time point. Theoptimal time point is defined by the maximum degree of efficiency. Witha reduction of the degree of efficiency, the exhaust gas is hottercompared to the operation without a deterioration in the degree ofefficiency. Accordingly, the exhaust gas develops an intensified heatingaction in the catalytic converter.

[0008] For engines having gasoline-direct injection, there exist, inprinciple, two possibilities to increase the exhaust-gas temperaturewithout adding additional components:

[0009] 1. Retarded ignition to deteriorate the degree of efficiency ofcombustion. The ignited mixture is stoichiometric or slightly lean.

[0010] 2. Additional injection of fuel after ignition for follow-oncombustion. The ignited mixture is very lean (stratified operation).

[0011] With the increasing rough running, the retarded ignition islimited for a homogeneous mixture. The emissions can furthermore beimproved by a slightly lean exhaust-gas lambda at low catalyticconverter temperatures. A leaning is, however, only possible to alimited extent for a cold engine.

[0012] If a secondary injection is provided for the catalytic converterheating, then the complete combustion of the additional fuel mass has tobe ensured. In order to ensure a reliable and complete combustion in theexhaust manifold, the latter must be optimized in its configuration withrespect to through mixing and low thermal mass. Other targets, such asthe reduction of structural space and the optimization of power canthereby be limited. In principle, the after-reaction takes place to apoorer degree in a cold exhaust manifold. Accordingly, the emissions canhardly be reduced shortly after the start.

[0013] Because higher temperatures are present in the combustionchamber, low emissions can be achieved already shortly after start withan after burning in the combustion chamber. If the fuel is still to beignited in the combustion chamber, then the operating parameters must beheld within a narrow window. Especially, the injection must start veryearly and therefore contributes significantly to the torque development.Very short injection times are a precondition for small load pointswhich implies very high demands on the injection valves.

[0014] The mixture preparation changes with the subdivision or splittingup of the injection in advance of ignition. With this type of mixture,the engine running can be improved. Basically, for a poorer degree ofefficiency and therefore a retarded ignition time point and a higherexhaust-gas temperature, an improved rough running is achievable and themixture can be more greatly leaned earlier after the start than for ahomogeneous mixture by simple injection. In this way, lower emissionsarise.

[0015] However, the accuracy of injection valves at small quantities isvery poor. For this reason, a subdividing of the injection for smallerair charges is not possible.

[0016] In order to ensure a reliable start and run-up of the engine,that is, of the internal combustion engine, a simple homogeneousinjection can still be necessary for this phase. A subdivision of theinjection takes place only when there is a sufficient air charge. Inthis way, short injection times are avoided which would lead to animprecise fuel metering.

[0017] Because of the subdivision of the injection, a mixturestratification arises. In this way, a rather rich mixture can be presentat the spark plug while the lambda sum is still lean. A reliableignition even for a lean lambda sum is ensured by the rich mixture aboutthe spark plug.

[0018] Notwithstanding late ignition, a reliable rapid ignition of themixture can additionally be ensured whereby the quiet running with lateignition is improved.

[0019] A different mixture distribution adjusts with divided injectiontaking place in advance of ignition, namely, rich in the center of thecombustion chamber and lean on the wall of the combustion chamber. Forthis reason, the wall heat loss can be reduced. Depending upon thecombustion chamber form and the parameters, the following effects canresult:

[0020] (i) a higher exhaust-gas temperature with the same exhaust-gasquantity and therefore more heating power for the catalytic converter;

[0021] (ii) a low exhaust-gas quantity at the same temperature becausethe wall heat losses are lower whereby the dwell times of the toxicsubstance components in the exhaust manifold and in the catalyticconverter become longer and an after-reaction is required. The emissionsafter the catalytic converter can therefore also be improved hereby.

[0022] Basically, at least once in the start phase, there must be aswitchover from the simple homogeneous injection (start and run-up) tothe divided injection (heat up of the catalytic converter) and back.Since the ratio for the injection quantity cannot be varied or can bevaried only slightly, there must be a hard switchover between these twotypes of mixtures.

[0023] Here, it can happen that the driver perceives the switchover,which can include an abrupt change in torque, as a jolt in the motorvehicle. The torque development is very different for simple homogeneousinjection and subdivided injection because of different mixture typesand the different combustion speeds. For this reason, the ignition timepoint must be abruptly shifted with the switchover and the air chargemust be rapidly changed. Even when the torque development for thischange is precisely modulated, inaccuracies result because of tolerancesof sensors and actuators, for example, via the imprecise detection ofthe air charge and the crankshaft angle.

[0024] Furthermore, inaccuracies of the fuel injection are also presentbecause two short injection times compared to one long injection timeare given. In this way, lambda deviations can additionally occur. Thisproblem can be eliminated only by more accurate injection valves.

[0025] The invention provides a method wherein a jolt-like change of theadjusting parameters and therefore of the torque can be reduced whilesimultaneously having improved heating of the catalytic converter.

[0026] The invention solves this task by a previously described methodwherein the first injection time point essentially corresponds to theinjection time point of the homogeneous operating mode during theswitchover operation from the homogeneous mode to the operating modewith subdivided injection and the second injection time point of thesubdivided injection takes place so early that the mixture arisinghereby corresponds approximately to a homogeneous mixture and, after thecompleted switchover, the second injection time point is shifted toretard until a pregiven mixture preparation is present and, for aswitchover from the operating mode with the subdivided injection to thehomogeneous operating mode, the shift of the second injection time pointreverses, that is, takes place in the direction of the first injectiontime point.

[0027] In this way, the mixture preparation can be switched over in sucha manner that the torque development of homogeneous and subdividedinjection is still similar. In this way, possible inaccuracies no longerlead to a conceivable torque jump. Only after the switchover thesubdivided injection is changed continuously until the wanted mixturepreparation is achieved.

[0028] This shift can take place continuously or step-wise. Theindividual discrete steps can each be so selected that no torque jump isperceived by the driver.

[0029] The optimum here is the continuous displacement.

[0030] The task is also solved by a computer program, a controlapparatus (open loop and closed loop) as well as an internal combustionengine in accordance with the claims.

[0031] Because the second injection time point lies close to the firstinjection time point directly after the switchover, the mixturecorresponds, shortly after the switchover to an operating mode withsubdivided injection, approximately to a homogeneous mixture havingindividual injection. Since the switchover between simple homogeneousinjection and subdivided injection always takes place with a secondinjection, which lies very early (that is, close to the firstinjection), the ignition time point and the air charge have to beadapted only minimally directly after the switchover. After theswitchover to the subdivided injection, the second injection time pointis then shifted to late, that is, to the actual desired value. In thisway, an adaptation of the air charge quantity takes place and, in theopposite case, an adaptation of the air charge quantity takes place inadvance of the switchback. In this way, as a rule, the air chargequantity is raised for the switchover to an operating mode withsubdivided injection. Furthermore, an adaptation of the ignition timepoint can be necessary.

[0032] The invention especially relates to a method wherein, in advanceof switchover, a check is made as to whether the air charge quantity inthe combustion chamber exceeds a pregiven limit value. This is necessaryinsofar that the accuracy of the fuel metering is reliably given onlystarting with certain injection quantities. With the subdivision of theinjection, the accuracy of the fuel metering is changed because now twoshort injection times are present compared to one long injection time.It is necessary that at least mean air charges are present and both fuelquantities must be approximately of the same magnitude. Only when thelarge air charges are reached, can the first injection quantity bevaried compared to the second injection quantity in the subdividedinjection.

[0033] Furthermore, it can be provided that the shift of the second timepoint is continuous or takes place in several separate discrete steps. Acontinuous shift with a continuous shift of the ignition time point aswell as of the air quantity is especially preferred.

[0034] Furthermore, it can be provided that the ignition time point isshifted in order to change the degree of efficiency after the switchoverfrom the homogeneous operating mode into the operating mode havingsubdivided injection and/or before the switchback with the displacementof the second injection time point.

[0035] Especially for a subdivided injection, a retarded ignition timepoint is possible whereby a deteriorated degree of efficiency can beachieved which, on the other hand, leads to an improved heating up ofthe catalytic converter. Notwithstanding the deteriorated degree ofefficiency because of the retarded ignition time point, the smoothrunning is, however, improved in an operation with subdivided injection.

[0036] Finally, the invention includes a computer program which issuitable for executing the method described above when it is run on acomputer. The computer program can be stored especially on a memory,especially a flash memory.

[0037] Furthermore, the invention relates to a control apparatus (openloop and/or closed loop) for operating an internal combustion enginewith the control apparatus including a memory on which a computerprogram as described above is stored. A control apparatus of this kindfunctions for controlling all operations in the engine, for example, themetering of the particular injection quantities, adjusting the ignitiontime points, metering of the corresponding air quantities, et cetera.

[0038] Finally, the invention also includes an internal combustionengine comprising: a combustion chamber; a fuel injection device viawhich fuel reaches the combustion chamber; a control apparatus (openloop and/or closed loop); a catalytic converter wherein, for heating upthe catalytic converter, a switchover is provided between a homogeneousoperating mode with a one-time injection and an operating state withsubdivided injection of fuel at at least two injection time points intoa combustion chamber of the internal combustion engine; for thesubdivided injection, both injection time points lie ahead of anignition of the air/fuel mixture; directly after the switchoveroperation from the homogeneous operating mode to the operating mode withsubdivided injection, the first injection time point correspondsessentially to the first injection time point of the homogeneousoperating state and the second injection time point of the subdividedinjection at first is so close to the first injection time point thatthe arising mixture corresponds approximately to a homogeneous mixtureand the second injection time point then can be displaced toward retardaway from the first injection time point until a pregiven mixturepreparation is present; and, the second injection time point isdisplaceable in the opposite direction for a switchover from theoperating state with subdivided injection to the homogeneous operatingstate.

[0039] In total, with the subdivision of the injection, the followingadvantages are achieved because here another type of mixture is present:

[0040] the motor running is improved;

[0041] a deteriorated degree of efficiency is possible for an improvedsmooth running (retarded ignition time point); and,

[0042] the mixture can be more greatly leaned.

[0043] At the same time, abrupt changes of the adjusting parameters canbe avoided by the described switchover strategy. The mixture preparationis switched over in such a manner that the torque developments ofhomogeneous injection and subdivided injection are so similar thatinaccuracies (for example, because of tolerances of sensors andactuators) no longer lead to a perceivable abrupt change in torque.Furthermore, no torque losses with switchover are perceivable which thedriver perceives in the form of a jolt.

[0044] Further features, application possibilities and advantages of theinvention become apparent from the description of an embodiment of theinvention which follows and which is shown in the figure of the drawing.All described or shown features by themselves or in any desiredcombination define the subject matter of the invention independently oftheir summary in the patent claims or their dependency as well asindependently of their formulation and showing in the description anddrawing, respectively.

[0045]FIG. 1 shows an internal combustion engine; and,

[0046]FIG. 2 shows traces of parameters in the switchover with a shiftof the second injection time point and for a spontaneous switchover.

[0047]FIG. 1 shows an internal combustion engine 1 of a motor vehiclewherein a piston 2 is movable back and forth in a cylinder 3. Thecylinder 3 is provided with a combustion chamber 4 which, inter alia, isdelimited by the piston 2, an inlet valve 5 and an outlet valve 6. Anintake manifold 7 is coupled to the inlet valve 5 and an exhaust-gaspipe 8 is coupled to the outlet valve 6. An injection valve 9 and aspark plug 10 project into the combustion chamber in the region of theinlet valve 5 and the outlet valve 6. Fuel can be injected into thecombustion chamber 4 via the injection valve 9. The fuel in thecombustion chamber 4 can be ignited by the spark plug 10.

[0048] A rotatable throttle flap 11 is accommodated in the intakemanifold 7 via which air is supplied to the intake manifold 7. Thequantity of the supplied air is dependent upon the angular position ofthe throttle flap 11. A catalytic converter 12 is accommodated in theexhaust-gas pipe 8 and functions to purify the exhaust gases developedby the combustion of the fuel.

[0049] An exhaust-gas recirculation pipe 13 leads from the exhaust-gaspipe 8 back to the intake manifold 7. An exhaust-gas recirculation valve14 is accommodated in the exhaust-gas recirculation pipe 13 with whichthe quantity of the exhaust gas, which is recirculated into the intakemanifold 7, can be adjusted.

[0050] A tank-venting line 16 leads from a fuel tank 15 to the intakemanifold 7. A tank-venting valve 17 is accommodated in the tank-ventingline 16. The quantity of fuel vapor supplied to the intake manifold 7from the fuel tank 15 can be adjusted with the tank-venting valve 17.

[0051] A back and forth movement is imparted to the piston 2 by thecombustion of the fuel in the combustion chamber 4. This movement istransmitted to a crankshaft (not shown) and applies a torque thereto.

[0052] Input signals 19 are applied to a control apparatus 18 for openloop control and/or closed loop control. The input signals 19 representoperating variables of the internal combustion engine 1 which aremeasured by sensors. For example, the control apparatus 18 is connectedto an air mass sensor, a lambda sensor, an rpm sensor and the like.Furthermore, the control apparatus 18 is connected to an acceleratorpedal sensor which generates a signal which indicates the position of anaccelerator pedal, which is actuable by a driver, and therefore therequested torque.

[0053] The control apparatus 18 generates output signals 20 with whichthe performance of the internal combustion engine 1 can be influencedvia actuators or positioning devices. For example, the control apparatus18 is connected to the injection valve 9, the spark plug 10 or thethrottle flap 11 and the like and generates the signals required todrive the same.

[0054] The control apparatus is, inter alia, provided for controlling(open loop and/or closed loop) the operating variables of the internalcombustion engine 1. Especially, the fuel mass, which is injected by theinjection valve 9 into the combustion chamber 4, is controlled by thecontrol apparatus 18 especially in view to a low fuel consumption and/ora low development of toxic substances. For this purpose, the controlapparatus 18 is provided with a microprocessor (computer) which has aprogram stored in a memory medium (especially in a flash memory) andwhich program is suitable for carrying out the above-mentioned control(open loop and/or closed loop).

[0055] The control apparatus 18 especially determines the throttle flapangle and the injection pulsewidth which define essential actuatingquantities for realizing the desired torque, the exhaust-gas compositionand the exhaust-gas temperature. The actuating variables are to bematched to each other. A further essential actuating variable forinfluencing these quantities is the angular position of the ignitionrelative to the piston movement.

[0056] In this context, the catalytic converter temperature can bedetermined. Here, measurements as well as also a modulating fromoperating quantities are considered. Especially at the start of theengine, the problem is, however, present that the catalytic converter 12does not have the adequate operating temperature. It is thereforenecessary that the catalytic converter 12 be brought as rapidly aspossible to the operating temperature after a cold start. This heat-upcan take place with the so-called motoric catalytic converter heatingvia a high exhaust-gas temperature.

[0057] The start of an engine takes place, as a rule, in a firstoperating mode, the so-called homogeneous operating mode of the engine1. The throttle flap 11 is partially opened or closed in dependence uponthe desired torque. The fuel is injected into the combustion chamber 4by the injection valve 9 during an induction phase caused by the piston2. With the air, which is inducted simultaneously via the throttle flap11, the injected fuel is swirled and therewith is essentially uniformlydistributed in the combustion chamber 4. Thereafter, the air/fuelmixture is compressed during the compression phase in order to beignited by the spark plug 10. With the expansion of the ignited fuel,the piston 2 is driven. The occurring torque is essentially dependentupon the position of the throttle flap 11 in the homogeneous operation.The throttle flap is essentially closed in the start phase. With a viewto a low toxic substance development, the air/fuel mixture is adjustedat lambda=1 or is adjusted slightly lean at lambda>1.

[0058] To increase the exhaust-gas temperature, it can be provided todeteriorate the degree of efficiency of the combustion in that theignition takes place at a later crankshaft angle. The ignited mixture isadjusted to be stoichiometric or slightly lean. However, for ahomogeneous operating mode, the disadvantage is present that the smoothrunning of the engine is not adequate.

[0059] According to the invention, the start of the internal combustionengine 1 nonetheless takes place in the homogeneous operating modebecause the air quantity during start run-up is not always adequate fora subdivided injection.

[0060] As soon as a conclusion can be drawn as to a sufficiently largeor at least mean air charge from the position of the throttle flap 11 orother sensor signals, a switchover into an operating mode withsubdivided injection is carried out for heating the catalytic converter.Initially, a first injection quantity is injected into the combustionchamber and a second injection quantity is injected at a latercrankshaft angle. The two injection time points lie ahead of theignition time point of the spark plug 10. With the subdivision of theinjection, a mixture stratification arises. A rather rich mixture ispresent at the spark plug 10 even though the lambda sum in the totalcombustion chamber 4 is still lean. A reliable ignition even for a verylean lambda sum is ensured by the rich mixture about the spark plug.Notwithstanding a retarded ignition, a reliable rapid ignition of themixture is additionally ensured. In this way, the smooth runningincreases even for retarded ignition and therewith a deteriorated degreeof efficiency. In this way, wall heat losses are reduced and higherexhaust-gas temperatures can be achieved for like exhaust gasquantities. In this way, the heating of the catalytic converter isachieved more rapidly.

[0061] With the switchover, the following problems are present: atfirst, the switchover can only take place when a minimum air charge ispresent because, otherwise, the fuel quantity per injection, which is tobe injected, is too low in order to ensure that the control of the fuelmetering has a sufficient accuracy. Furthermore, the torque developmentis very different for simple homogeneous injection and subdividedinjection because of the different mixture types and the differentcombustion speed. Accordingly, with the switchover, the ignition timepoint must be shifted abruptly and the air charge must be changedrapidly. Even when the torque development for these changes can bemodeled precisely, inaccuracies arise because of tolerances of thesensors and actuators. Accordingly, a torque jump can occur which thedriver perceives. Furthermore, lambda deviations can occur because theprecision of the fuel metering is different for the two types ofinjection.

[0062] According to the invention, it is therefore suggested that themixture preparation is reconfigured in such a manner that the torquedevelopment of homogeneous injection and subdivided injection is stillsimilar at the time point of the switchover. Here, a perceptible abruptchange in torque no longer occurs because of possible inaccuracies.After the switchover, the subdivided injection is continuously changed,that is, the two injection time points are spread apart in that thesecond injection time point is shifted to retard until the desiredmixture preparation is reached.

[0063] Directly after the switchover, the second injection time point isso early that it lies virtually at the first injection time point andtherefore the mixture approximately corresponds to the homogeneousmixture with single injection. The ignition time point and the aircharge must then only be adapted minimally. After the switchover, thesecond injection time point can be shifted continuously to the actualdesired value, that is, in the direction of the ignition time point.Ignition time point and air charge are then adapted to the changedmixture preparation and torque development. Especially, the air chargeis increased in order to counter a torque loss. The ignition time pointcan be shifted further rearwardly when there is uniform smooth runningbecause a divided injection permits a higher smooth running for adeteriorated degree of efficiency than a homogeneous injection.

[0064] In total, the mixture types have similar characteristics at theswitchover. Inaccuracies of sensors and actuators therefore do not actdifferently. Especially when the shift of the second injection timepoint of the second fuel quantity takes place slowly and continuously,inaccuracies are not perceptible any longer and they do not lead to areduction in the driving performance of the vehicle.

[0065] If the air charge is increased after or during the shift of thesecond fuel injection to “late”, the ignition time point is adapted incorrespondence to the higher charge and the changed mixture preparation.Overall, steps and jumps, which result in the maximum torque at optimalignition, can be brought close to each other in such a manner that theindividual operating modes pass approximately continuously one into theother.

[0066]FIG. 2 shows in detail how the individual parameters change whenthe switchover is changed abruptly or by continuous displacement of thesecond fuel injection with respect to the time point.

[0067]FIG. 2 shows, on the left-hand side, the maximum torque foroptimal ignition, the ignition time point with respect to the distanceahead of top dead center, the ignition time point with respect to thedistance to top dead center and the air charge for a direct switchoverto the wanted desired value for the subdivided injection. The right-handillustrations show, in contrast, a “ramping” in accordance with theinvention. The injection time point of the second fuel quantity at firstlies close to the first ignition time point and, therefore, the mixturecharacteristic corresponds closely to that of the homogeneous injection.

[0068] It can be especially well seen that in the lowest diagram, whichshows the maximum torque at optimal ignition for an individual injectionover time, at first a constant torque is present. This torque increasesbecause even at optimal ignition for the subdivided injection, a lowertorque is developed than for a simple homogeneous injection andtherefore, more air charge must be built up already ahead of theswitchover. Therefore, the optimal torque increases for the homogeneousinjection with the air charge. This torque then drops abruptly at thetime point of the switchover which is shown in each case by the centerbroken perpendicular line. This power loss is perceived by the driver asa jolt if the power loss is not adequately compensated via thecorrection of the ignition time point. With the increase of the airquantity as well as the adjustment of an optimal ignition time point,this torque can slowly again be increased.

[0069] In a switchover, the second fuel injection is first also early,that is, far forward of the top dead center point and the mixture actslike a homogeneous mixture at first also after the switchover. With theshift of the ignition time point and the second injection time point,the optimal torque, in turn, increases to the end value. A jump is notapproximately to be determined via the switchover.

[0070] The second diagram from below shows the spacing from bottom deadcenter with respect to the second injection time point. While it isshown on the left side that this injection time point is at a relativelyshort distance from top dead center already at the time point of theswitchover, it can be seen in the right-hand diagram and that thisinjection time point, at first, shows a considerable distance from topdead center after the switchover and then only is moved during theswitchover toward the desired value which lies close to top dead center.

[0071] It can likewise be seen that, at first, the ignition time pointfor a homogeneous injection has a relatively large distance from topdead center, that is, an early ignition takes place because only then isa good smooth running ensured for homogeneous injection. When switchoveris approached, this time point is displaced rearwardly in order, for theincreased air charge, which results from the uppermost diagram, tocounter a further increase of the torque for homogeneous operating mode.By shifting the ignition time point rearwardly, the smooth running is,however, deteriorated.

[0072] After the switchover, the ignition time point has to be againshifted abruptly to early in order to first counter a drop in torque foruniform air charge and thereby achieve an optimal ignition. At the latertime point, the ignition time point can again be shifted rearwardly. Ashift can take place overall significantly farther rearwardly, that is,the ignition can take place at a later time point than in a homogeneousmode of operation because here the smooth running is not affected by thedeteriorated degree of efficiency.

[0073] If one now views the right diagram, it can be seen that here too,the ignition time points only carry out a small jump because, here too,the mixture characteristics are similar at the time point of theswitchover.

[0074] The air charge is approximately the same for the abruptswitchover as well as for the continuous switchover because this aircharge must be increased for an operating mode with divided injection inorder to obtain an optimal torque.

1. Method for operating an internal combustion engine, especially formotor vehicles, the engine having a catalytic converter for heating upthe catalytic converter, wherein there is a switchover between ahomogeneous operating mode with one-time injection of fuel into acombustion chamber of said engine and an operating mode with subdividedinjection of fuel at at least two injection time points into saidcombustion chamber of said engine, wherein, for subdivided injections,both injection time points lie ahead of an ignition of an air/fuelmixture, characterized in that the first injection time pointessentially corresponds to the injection time point of the homogeneousoperating mode during the switchover operation from the homogeneousoperating mode to the operating mode with subdivided injection, and thesecond injection of the subdivided injection at first takes place soearly that the arising mixture corresponds during operation withsubdivided injection approximately to a homogeneous mixture and, afterthe completed switchover, the second injection time point is shiftedtoward late until a pregiven mixture preparation is present and, for aswitchover from the operating mode with the subdivided injection to thehomogeneous operating mode, the shift of the second injection time pointtakes place in the opposite direction.
 2. Method of claim 1,characterized in that, in advance of the switchover, a check is made asto whether the air charge quantity in the combustion chamber exceeds apregiven limit value.
 3. Method of claim 1 or 2, characterized in that,after the switchover from homogeneous operating mode into the operatingmode with subdivided injection, the air charge quantity is increasedwhen the second injection time point is shifted toward late; and, theadaptation of the air charge quantity in advance of the return switchinginto the homogeneous operating mode takes place in a correspondinglyreversed manner.
 4. Method of one of the above claims, characterized inthat, after the switchover from the homogeneous operating mode into theoperating mode having subdivided injection and/or, in advance ofswitching back, the ignition time point is shifted for the shift of thesecond injection time point.
 5. Method of one of the above claims,characterized in that the shift of the second injection time point takesplace continuously or in several discrete steps.
 6. Method of one of theabove claims, characterized in that during the shift of the secondinjection to late, the injected fuel quantity is reduced in order toachieve an overall lean lambda.
 7. Method of claim 6, characterized inthat, with the shift of the second injection to early, the injected fuelquantity is again increased in order to maintain the lean running limitfor simple homogeneous injection.
 8. Computer program, characterized inthat the program is suitable for carrying out the method of one of theabove claims when it is carried out on a computer.
 9. Computer programof claim 8, characterized in that it is stored in a memory, especiallyin a flash memory.
 10. Control apparatus (open loop and/or closed loop)for operating an internal combustion engine, characterized in that thecontrol apparatus includes a memory on which a computer programaccording to one of the claims 8 or 9 is stored.
 11. Internal combustionengine comprising: a combustion chamber; a fuel injection device viawhich fuel reaches the combustion chamber; a control apparatus (openloop and/or closed loop); a catalytic converter; wherein, especially forheating up the catalytic converter, a switchover is provided between ahomogeneous operating mode with a one-time injection and an operatingmode with subdivided injection of fuel at at least two injection timepoints into the combustion chamber of the internal combustion engine;for the subdivided injection, both injection time points lie ahead of anignition of the air/fuel mixture; characterized in that, directly afterthe switchover operation from the homogeneous operating mode to theoperating mode with subdivided injection, the first injection time pointessentially corresponds to the injection time point of the homogeneousoperating mode and the second injection time point of the subdividedinjection at first is so close to the first injection time point thatthe hereby arising mixture corresponds approximately to a homogeneousmixture and the second injection time point then can be displaced towardlate away from the first injection time point until a pregiven mixturepreparation is present; and, the second injection time point isdisplaceable in the opposite direction for a switchover from theoperating mode with subdivided injection to the homogeneous operatingmode.
 12. Internal combustion engine of claim 11, characterized in thatthe engine includes a control apparatus (open loop and/or closed loop)of claim 10.